This invention generally relates to aqueous thickener polymer compositions, their method of manufacture and method of use.
Aqueous polymer systems, for example coatings containing emulsion polymer binders, typically use thickeners to obtain the desired degree of viscosity needed for the proper formulation and application of the aqueous system. One general type of thickener used in aqueous polymer systems is referred to in the art by the term “associative.” Associative thickeners are so called because the mechanism by which they thicken is believed to involve hydrophobic associations between the hydrophobic moieties in the thickener molecules and/or between the hydrophobic moieties in the thickener molecules and other hydrophobic surfaces. One type of commonly used associative thickener has a polymeric backbone constructed from one or more blocks of polymerized oxyalkylene units, typically polyethylene oxide or polypropylene oxide, with hydrophobic groups attached to or within the backbone. Another type of commonly used associative thickener utilizes a cellulosic backbone with hydrophobic groups attached to the backbone. Both of these types of associative thickeners can be characterized as polyether thickeners as they both have backbones comprising ether linkages. Known polyether associative thickeners are non-ionic thickeners, and their thickening efficiencies in aqueous systems are substantially independent of pH.
In addition to polyether segments, other types of segments can be incorporated into the backbone of a polyether associative thickener. Associative thickeners with polyurethane polyether backbone segments and containing hydrophobic groups comprising tertiary and secondary amine functionalities have been disclosed. U.S. Pat. No. 6,939,938 discloses associative polyurethane polyether thickeners with amine functional hydrophobic groups in which at least 85% of the amine functionality is converted to permanently cationic quaternary amine functionality. Because the quaternary amines are permanently cationic, the associative nature of the groups cannot be turned on and off readily by, for example, pH changes.
Most of the associative thickeners presently on the market are sold as pourable aqueous liquids. For ease of use, it is desirable for the viscosity (Brookfield at 6 rpm) of such thickener products to be less than 15,000 centipoise (cps), or even less than 5,000 cps, so that the product will readily drain from its storage container, and be readily incorporated into the aqueous system to which it is added. The viscosity of the aqueous thickener product can be decreased by reducing the active solids concentration, but this has the drawback of limiting formulation latitude in terms of weight solids of the aqueous system to be thickened by the product.
Other techniques for lowering polyether associative thickener viscosity are also unsatisfactory. Mixtures of polyether associative thickener and water with water miscible, organic co-solvents such as diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, ethylene glycol, polyethylene glycol, propylene glycol or polypropylene glycol, have been used. However, use of these volatile organic solvents is contrary to the need to meet ever more stringent environmental regulations, including the reduction of Volatile Organic Content (VOC). Thus, although the organic co-solvents perform their intended role, they possess potential environmental, safety and health disadvantages. Another method to suppress the product viscosity of polyether associative thickeners is the admixture of surfactants with the aqueous associative thickener. However, the relatively high level of surfactant required can negatively impact the thickening efficiency of the thickener product in the aqueous system to be thickened, and it can degrade final dried coating properties. In addition, the surfactant adds cost to the product.
The admixture of cyclodextrin compounds with the aqueous thickener product to suppress viscosity has also been disclosed. The cyclodextrin suppresses the viscosity of the polyether thickener product until the product is added to an aqueous system containing levels of surfactant high enough to displace the thickener hydrophobe from the cyclodextrin cavity. The primary disadvantage of this method has been the high cost of cyclodextrin compounds.
High product viscosity KU building polyether thickeners can be blended with low product viscosity, ICI building polyether thickeners to provide a blend at an intermediate viscosity. However, when using this method, the flexibility to thicken to different KU and ICI viscosity targets in different coating formulations is compromised.
Another approach to providing an aqueous thickener at high active solids concentration in water is the use of hydrophobically modified alkali swellable or soluble emulsion (HASE) thickeners. HASE thickeners rely upon the insolubility of the polymer backbone itself at low pH to allow the thickener to be provided at low viscosity and at relatively high solids in the emulsion form. However, the substantially ionic backbone ultimately generates other problems related to water sensitivity in the applied coating.
A need in the art remains, therefore, for pourable associative thickeners with both low viscosity and the highest active thickener solids possible. A particular need exists for a cost-effective, environmentally friendly method to suppress the aqueous product viscosity of polyether associative thickeners with practical active solids concentration.